Outer Limits 4x4 Board

My father has an 80 series with an EFI 3F. It has a missfire problem occasionaly. It comes and goes. Does it hot and cold. And at any speed. It seams like it drops 1 or 2 cyl and when it does this at speed the whole car shudders. It doesnt seam to do it at idle very often.

Anyone had this problem with a 3F? Anyone know how to fix it. Allready done leads and plugs and its no better.

Have you checked something easy like leaks after the AFM.
My 80 had a small crack in the pipe and it did wierd things as the pipe would move and let air in.